1. Technical Field
The present disclosure relates to a wiring substrate and a method of manufacturing the wiring substrate, and more particularly, to a wiring substrate including a core layer where wiring patterns are formed on both surfaces of a core substrate made of a conductive material via insulating layers and are connected to each other by a via formed in a through hole passing through the core substrate, and a method of manufacturing the wiring substrate.
2. Related Art
In recent years, a core substrate, which is mainly made of carbon fiber and an insulating resin and has a thermal expansion coefficient lower than other members of a wiring substrate, (hereinafter, simply referred to as a core substrate having a low thermal expansion coefficient) has been used in order to reduce a thermal expansion coefficient of the wiring substrate (see e.g., International Patent Publication No. WO2004/064467).
However, in the above core substrate, conductive carbon fiber is used in the core substrate having the low thermal expansion coefficient. For this reason, as shown in FIG. 7, in a core layer 102 constituting a wiring substrate 100, a via 106 is formed in a through hole 104 passing through a core substrate 102a with an insulating layer 102b interposed between the inner surface of the through hole and the via. The via electrically connects wiring patterns 102c and 102c that are formed on both surfaces of the core substrate 102a, which has a low thermal expansion coefficient, via the insulating layer 102b made of an insulating resin. The via 106 is a cylindrical via 106a formed in the through hole 104 via the insulating layer 102b, and a hollow portion of the cylindrical via 106a is filled with a resin 106b. 
In the wiring substrate 100 shown in FIG. 7, wiring patterns 110 are formed on both surfaces of the core layer 102 with insulating layers 108 interposed therebetween.
In order to manufacture the wiring substrate 100 shown in FIG. 7, firstly, as shown, in FIG. 8A, the through hole 104 is formed in the core substrate 102a, which is made of a carbon fiber and an insulating resin and has a low thermal expansion coefficient, by a drill. Then, as shown in FIG. 8B, the insulating layer 102b is formed on both surfaces of the core substrate 102a with an insulating resin, and the through hole 104 is filled with the insulating resin.
Further, as shown in FIG. 8C, a through hole 105 having a diameter smaller than that of the through hole 104 is formed in the through hole 104 of the core substrate 102a by a drill. Then, a metal layer 114 is formed on the surfaces of the insulating layers 102b and on the inner surface of the through hole 105 by plating or vapor deposition (see FIG. 8D).
As shown in FIG. 8E, the through hole 105 is filled with the insulating resin 106b. Then, protrusions of the insulating resin 106b, which protrude from the through hole 105, are removed by performing polishing so that the insulating resin is flush with the surface of the metal layer 114 (see FIG. 8F).
As shown in FIG. 9A, a metal layer 116 is formed on the surface of the metal layer 114 and the polished surfaces of the insulating resin 106b by plating deposition. Then, the wiring patterns 102c are formed by patterning the metal layers 114 and 116 so that the core layer 102 is formed (see FIG. 9B).
In the core layer 102, the via 106 is formed in the through hole 104, which passes through the conductive core substrate 102a, with the insulating layer 102b interposed therebetween. The via 106 is the cylindrical via 106a formed in the through hole 104 with the insulating layer 102b interposed therebetween, and a hollow portion of the cylindrical via 106a is filled with the insulating resin 106b that forms the insulating layer 102b. 
Then, the wiring patterns 110, 110 . . . are layered on the both surfaces of the core layer 102 by a known additive method or semi-additive method via the insulating layers 108. Accordingly, it is possible to form the wiring substrate 100 shown in FIG. 7.
Further, a via formed by inserting a coaxial wire, where the surface of a resin film covering a metal wire is covered with plated metal, into a through hole of a substrate is described in JP-A-2007-12746. According to this method, it is possible to simply form a via.
However, in the case where the coaxial wire is inserted into the through hole, alignment of the coaxial wire is necessary and the metal wire of the coaxial wire needs to be exposed in a longitudinal direction of the through hole in this method. For this reason, the steps thereof become complicated. Furthermore, the coaxial wire needs to have a certain level of strength so that the coaxial wire can be inserted into the through hole. For this purpose, if the coaxial wire is made thin in order to form a via with a small pitch, the buckling or separation of the coaxial wire is likely to occur.
A core substrate, which is made of a carbon fiber and an insulating resin and has a thermal expansion coefficient lower than other members of the wiring substrate, is used as the core substrate 102a in the wiring substrate 100 shown in FIG. 7. For this reason, the thermal expansion coefficient of the wiring substrate can be lowered as compared with the related-art wiring substrate that is formed using a core substrate made of a resin. Accordingly, even when a semiconductor device is mounted on the wiring substrate 100 shown in FIG. 7, it is possible to decrease the generation of a crack caused by the difference between the thermal expansion coefficients of the semiconductor device and the wiring substrate.
However, in the wiring substrate 100 shown in FIG. 7, the via 106 passing through the core substrate 102a is formed in the through hole 104 formed in the core substrate 102a with an insulating layer 102b, which is formed along the inner surface of the through hole, interposed therebetween.
For this reason, in the steps of manufacturing the wiring substrate 100 shown in FIG. 7, after the through hole 104 is formed to pass through the core substrate 102a, the through hole 104 is filled with an insulating resin and then the through hole 105 needs to be formed again. The through hole 105 should be accurately formed at the center of the through hole 104. This is because there is a possibility that the thickness of the insulating layer 106a has deviation and the reliability of the via 106 deteriorates if the through hole 105 is deviated from the center of the through hole 104.
Further, in order to ensure the reliability of the via 106 by reducing the deviation of the thickness of the insulating layer 106a as much as possible, a certain level of the diameter of the through hole 104, which is to be formed in the core substrate 102a, should be ensured in view of the accuracy of processing for forming the through hole 105 in the through hole 104. Thus, it is difficult to form the via 106 with a small pitch.